1. Field of the Invention
The present invention relates, generally, to dual clutch transmissions and, more specifically, to dual clutch transmissions having an area controlled hydraulic circuit used for governing the flow of cooling fluid provided to each of the two clutches of a dual clutch transmission.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. One type of transmission that has been proposed for use in conventional motor vehicles includes automated manual-type transmissions.
Some automated manual type transmissions can be power-shifted to permit gearshifts to be made under load. Automated manual transmissions having two clutches are generally referred to simply as dual, or twin, clutch transmissions. The dual clutch structure is most often configured so as to derive power input from a single engine flywheel arrangement. However, some designs have a dual clutch assembly having different input sources. Regardless, the layout is the equivalent of having two transmissions in one housing, namely one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each transmission can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmissions may include two wet or dry disc clutches each with its own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electro-mechanical type, a wet clutch system requires a lubrication system including a pump. Dual clutch transmissions of this type utilize hydraulic shifting and clutch control. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform up-shifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear engagement in a dual clutch transmission is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. To automate the transmission, the mechanical selection of each of the gear sets is typically performed by some type of actuator that moves the synchronizers. A reverse gear set includes a gear on one of the input shafts, a gear on the counter shaft, and an intermediate gear mounted on a separate counter shaft meshingly disposed between the two so that reverse movement of the output shaft may be achieved. In some dual clutch transmissions, the synchronizers are located on the countershafts, due to the arrangement of the coaxial input shafts and countershafts and the desire to minimize the length of the transmissions. This arrangement can permit the pinions on the input shafts to be more closely spaced than if the synchronizers were located on the input shafts and between the pinions. However, dual clutch transmissions of this type suffer from certain disadvantages.
For example, each of the synchronizers must have a capacity sufficient to transfer torque between the countershaft and the selected gear located on the countershaft. The diameters of the gears vary depending upon the desired gear ratio. In first gear, it is often desirable to provide the countershaft with a reduction in the rotational speed and with an increased torque relative to the torque and speed of the input shaft. In order to accomplish this, the first gear pinions often have a relatively small diameter, which is limited by the diameter of its respective input shaft. The corresponding first gear on the countershaft is necessarily of very large diameter. The second gear pinion located on the even input shaft typically has a larger diameter than the first gear pinion, and the second gear located on the countershaft typically has a smaller diameter than the second gear, and so on.
Due to the comparatively large reduction in speed and increase in torque desired for the first gear ratio, the synchronizer torque capacity for the first gear on the countershaft often must be significantly greater than the synchronizer capacities for the other gears. Because the synchronizer for first gear is mounted on the countershaft, the synchronizer must have sufficient torque capacity to compensate for the additional torque load imposed by the relatively high gear ratio and rotational inertia for first gear. In general, the more capacity that is required for the synchronizer, the larger and more costly the synchronizer is. Therefore, in order to minimize the cost of such transmissions, a variety of different synchronizers having different capacities are used. For example, the synchronizers for first gear typically are larger and more costly than the synchronizers for the other gears, and may be of a different more complex construction, such as a multi-cone synchronizer. These more complicated, more costly synchronizers also raise additional durability and service issues.
In addition, the arrangement of the pinions and gears limits the minimum diameters (perpendicular to the axis of the input shafts) and the minimum lengths (parallel to the axis of the input shafts) of the transmission. For example, the diameter of the first gear, which is typically the largest gear diameter, is often a factor that limits efforts to reduce the diameter of the overall transmission. In addition, the number of gears on the countershaft can be a limiting factor on the minimum length of the transmission, because the gears are typically aligned in series, with a separate countershaft gear provided for each of the gears of the input shaft for the different gear ratios.
Accordingly, there remains a need in the art for a dual clutch transmission that has an even more simplified construction, reduces expensive components, and facilitates a smaller packaging envelope that allows the transmission to be employed in even smaller spaces.
In order to provide sufficient cooling to the clutch assemblies of the conventional dual clutch transmission, the clutch assemblies are usually bathed in transmission fluid in a generally uncontrolled manner. While dual clutch transmissions have overcome several drawbacks associated with conventional transmissions and the newer automated manual transmissions, it has been found that controlling and regulating the automatically actuated dual clutch transmission to achieve the desired vehicle occupant comfort goals is a complicated matter. There are a large number of events to properly time and execute within the transmission for each shift to occur smoothly and efficiently. In addition, the clutch and complex gear mechanisms, working within the close confines of the dual clutch transmission case, generate a considerable amount of heat.
Accordingly, there remains a need in the related art for an improved hydraulic circuit to provide cooling fluid and control to the clutch assemblies of the dual clutch transmissions. Specifically, there is a need for a dual clutch transmission having an improved hydraulic control that has a reduced complexity resulting in lower cost and a smaller packaging envelope while still maintaining good operational characteristics.